This invention relates to programmable circuits, and more particularly, to programmable circuits that adjust gain with respect to a given circuit impedance or a given circuit bandwidth.
Operational transconductance amplifier (OTA) circuits can be used to form active filters with voltage-variable control over a number of filter parameters. Such active filters are commonly called operational transconductance filters. An operational transconductance filter has the ability to change bandwidth and gain while preserving the shape of its attenuation versus frequency response. The transconductance of the operational transconductance filter can also affect the bandwidth of the passband.
A significant drawback of operational transconductance filters, however, is that a change in bandwidth often causes an undesired change in input impedance. Thus, for a given signal input current value, the input voltage to an operational transconductance filter changes when there is a change in bandwidth. This undesired change in voltage gain can be detrimental to systems that process a wide range of input signals.
For example, many receivers, such as direct-conversion or zero intermediate-frequency receivers, use a voltage-to-current circuit to convert the voltage output of a mixer to a current. This current drives the input of a baseband filter. Because the gain of voltage-to-current circuits often does not change with the input impedance of the baseband filter, the voltage gain of the cascade of the voltage-to-current circuit and the baseband filter is often proportional to the input impedance of the baseband filter. This relationship can be undesirable in applications that have a varying bandwidth. This condition may cause a loss of sensitivity for high frequency bandwidths (low Rgm/xcex1) or cause baseband filter input saturation for low frequency bandwidths (high Rgm/xcex1) where xcex1/Rgm is the transconductance (gm) of an operational transconductance amplifier. Thus, it is a challenge to maintain a stable voltage gain to ensure that an operational transconductance filter receives a sufficient portion of the signal strength of an input signal.